The present invention relates to noise reduction in a capacitive touch sensor for detecting proximity and location of a body, more especially to a one- or two-dimensional capacitive sensor of the so-called active type which is based on measuring the capacitive coupling between two electrodes at each sensing node in a detector array.
There are various forms of touch sensitive controls which use a capacitive sensor to sense the presence of a body such as a user's finger. A form of touch sensitive control is disclosed in WO-00/44018. In this example a pair of electrodes are provided which act as a key so that the presence of a body such as a user's finger is detected as a result of a change in an amount of charge which is transferred between the two electrodes. With this arrangement, one of the electrodes (labelled X) is driven with a drive circuit and the other of the pair of electrodes (labelled Y) is connected to a charge measurement circuit which detects an amount of charge present on the Y plate when driven by the X plate. As disclosed in WO-00/440018 several pairs of electrodes can be arranged to form a matrix of sensing areas which can provide an efficient implementation of a touch sensitive two-dimensional position sensor. Such two dimensional capacitive transducing (2DCT) sensors are typically used with devices which include touch sensitive screens or touch sensitive keyboards/keypads which are used in, for example, consumer electronic devices and domestic appliances.
Devices employing 2DCT sensors have become increasingly popular and common not only in conjunction with personal computers but also in all manner of other appliances such as personal digital assistants (PDAs), point of sale (POS) terminals, electronic information and ticketing kiosks, kitchen appliances and the like. 2DCT sensors are frequently preferred to mechanical switches for a number of reasons. For example, 2DCT sensors require no moving parts and so are less prone to wear than their mechanical counterparts. 2DCT sensors can also be made in relatively small sizes so that correspondingly small, and tightly packed keypad arrays can be provided. Furthermore, 2DCT sensors can be provided beneath an environmentally sealed outer surface/cover panel. This makes their use in wet environments or where there is a danger of dirt or fluids entering a device being controlled attractive. In addition, manufactures often prefer to employ interfaces based on 2DCT sensors in their products because such interfaces are often considered by consumers to be more aesthetically pleasing than conventional mechanical input mechanisms (e.g. push-buttons).
Other devices which may incorporate 2DCT sensors include pen-input tablets and encoders used in machinery for feedback control purposes, for example 2DCT sensors are capable of reporting at least a 2-dimensional coordinate, Cartesian or otherwise, related to the location of an object or human body part by means of a capacitance sensing mechanism.
Although touch sensitive capacitive sensors such as those described above and disclosed in the above-mentioned disclosures have been successfully deployed in many applications, some applications can present a challenging environment for detecting a change in charge as a result of the presence of a body.
For example, the use of a touch sensor on a mobile phone can create a technical problem because there is a variety of disturbing noise signals produced by radio frequency radiation, by radio frequency signals and by modulators within the mobile phone. Similarly, a liquid crystal display (LCD) has characteristic switching noise as a result of switching and refreshing pixels. Other types of display may have their own forms of characteristic impulsive noise related to pixel scanning and refresh. Sinusoidal noise, such as that produced by mains electricity may also be present, which can affect the amount of charge detected on a key. This may be significant, for example, when a hand held device such as a mobile telephone is being charged through the mains.
FIG. 7 of the accompanying drawings shows an example of sinusoidal noise in the form of a plot of signal strength or amplitude which may be voltage or charge measured with respect to time. As shown various points 220 are shown to indicate points at which burst measurements are taken for a touch sensor such as those described above. As will be appreciated, as a result of sinusoidal noise, the amount of charge transferred from a key by a measurement capacitor of the measurement circuit such as those described above will vary and therefore could in some circumstances cause a false measurement of the presence of a body.
FIG. 8 shows another form of noise, namely rectangular or impulsive noise, i.e. noise having high frequency components, such as that which might be produced by switching the pixels in a LCD display with which the touch panel is integrated. A plot of signal strength with respect to time is shown with sampling points 220, which might be produced by bursts of measurement cycles in a measurement circuit such as those described above. If a measurement cycle coincides with a rising edge of a noise impulse 222, as may arise from an LCD switching event, then an erroneous measurement can be produced which can again cause a touch sensor to erroneously detect the presence of a body.
FIG. 9 illustrates this situation showing simultaneous sinusoidal and rectangular noise. As will be appreciated, in general, both sinusoidal noise and rectangular noise may be present during a given time period. Moreover, by its nature, the amount of noise as well as its frequency components will often vary over time.
Prior art capacitive sensors adopt a variety of signal processing techniques to noise filter the acquired signals. For example, boxcar averagers and detection integrators have been used in the past. In principle other types of standard filtering could be used, and may have been used, such as slew rate filters, high frequency pass filters, low frequency pass filters and band pass filters.
However, choosing the appropriate filter is difficult in view of the inherent unpredictability and variability of noise. Even if the frequency of the main noise components can be predicted, their magnitude over time may be unpredictable. Moreover, in real time applications, such as touch data processing, any filtering needs to be rapid. On the other hand, with touch data processing, it is generally desirable for the signal to be pre-processed “on chip”, i.e. in the same chip as is used to receive the touch data from the touch panel, rather than at a higher system level, i.e. on a personal computer or other electronic system with a state of the art central processing unit. Typically, the raw touch data are initially processed by a microcontroller with severely limited numerical processing and memory specifications, but it is nevertheless desirable to ensure that the microcontroller takes measures to reduce noise in the touch data supplied to higher level system components.